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Abstract:

Several studies in temperate regions have observed an active behavioral response of stream invertebrates to environmental factors. However, few of these studies have been conducted in the tropics and the behavior of tropical stream invertebrates is unknown. In an attempt to improve the knowledge on tropical stream ecology, this study observed indirect behavioral responses to discharge within the drift as well as the
benthic macroinvertebrate community. Drift, benthic, and discharge samples were taken at 11 sites along the Quebrada Máquina from October 28-November 13 of 2004. The drift community richness and diversity and discharge resulted in a non-significant, negative trend. However, the benthic community richness and diversity and discharge resulted in a significant positive correlation. Further analysis showed that drift and benthic communities were similar in composition, implying that the drift community arises directly from the benthic community. The positive correlation of the benthic community to discharge suggests that, similar to temperate macroinvertebrates, tropical macroinvertebrates actively respond to discharge. ( , )

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2 Numerous studies have been performed on temperate streams to test active behavioral response by studying the factors that affect drift communities. Kohler (1985) supported the active response by demonstrating that macroinvertebrates respond to low light and food levels. Koetsier and Bryan ( 1995) tested the abiotic factors that cause a response in the benthic and drift community by comparing the effect of discharge on drifting communities in the Mississippi River, Louisiana. They found that several drifting taxa, including Ephemoptera and Tr ichoptera, were significantly, negatively related to discharge. In addition, they found a positive correlation in abundance be tween discharge and benthic communities. They proposed that an increase in discharge leads to an increase in surface area macroi nvertebrates can colonize, and therefore a decrease in competition for space. This supports the hypothesis that macroinvertebrates actively drift. However, they also noticed that some drift organisms spent their entire life cycle drifting. This suggests that the two communities are different, which weakens their argument on drift being an active behavioral response from the benthic community (Smock 1996). Perhaps because of the high daily variability in tropical streams due to the high amounts of preci pitation in the tropics , few researchers have attempted to study the behavior of tropical drift communities (Masters 2004) . In fact, there has never been a study on the effects of discharge on the benthic and drift communities. Yet understanding the beha vior of the macroinvertebrate community is a crucial element to understanding stream ecology. Moreover , the current poor state of knowledge of freshwater invertebrates makes assessing anthropogenic disturbances difficult (Palmer and Lake 2001). This stud y attempts to improve the knowledge of a tropical stream. Based on past studies of temperate streams, this study hypothesized that an area of tropical stream with high discharge should have a lower drift of macroinvertebrates and a greater amount of benth ic macroinvertebrates than that of an area of stream with lower discharge. Furthermore, I expected the drift and benthic communities to have different species compositions. MATERIALS AND METHODS Study Area Data were collected between five and 55 meters above the dam on Quebrada MÃ¡quina, Montverde, Costa Rica (Figure 1). The stream was chosen to minimize the effects of other factors, like pollution, that are known to cause drift. The unpolluted stream is located in the upper reaches of the Lower Pre Mo ntane Wet Forest. The sites were located between the altitudes of 1460 and 1465 meters (Holdridge 1967). Samples were taken for a total of 11 sites (one site/day) starting at 10:00 AM each day from October 28 November 13 , 2004. Sites were set at five meter intervals (Figure 2). To achieve the best sample of each community, drift nets were placed in the riffle stretches of the stream. Benthic samples were taken from highly heterogeneous substrate because a past study performed by Hyman (2002) showed gr eater community diversity as heterogeneity increased. High substrate heterogeneity was determined visually.

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3 Sampling Daily measurements included discharge, dissolved oxygen, drift, and benthic kick. A ~100Âµm net was placed halfway into the water for a total of 30 minutes to measure drift. Benthic kick is a process in which the stream substrate is stirred up and then captured downstream by a kick net. There were two one minute, one square meter benthic kicks per site within two meters of the placement of the drift net. The velocity of the stream was determined by the float method in which an orange was placed in the stream and the time it took to travel ten meters downstream was recorded A meter stick was used to measure the cross sectional area. D ischarge was calculated by multiplying the velocity by the cross sectional area. Dissolved oxygen was measured using Oakton DO 300 series O 2 meter (serial number WD 35615 75). Both benthic and drift samples were identified the day of sampling under a ste reomicroscope. Identification to Order and Family was performed using the dichotomous key titled How to know the Aquatic Insects (Lehmkuhl 1979). Analysis Sites differed in discharge, and therefore the number of individuals may differ solely due to wate r volume differences. To factor out this variation in the water volume sampled, the total number of individuals/site was divided by the total volume of water passing through the net over the 30 minute period. Benthic sample sizes were not altered because sample size was not subject to discharge. Family and Order richness for the drift samples were also volume corrected. The Shannon Weiner index of diversity (H') was used to calculate Family and Order diversity for both drift and benthic samples. Spearm an rank correlations were used to test the relation of the rate of discharge with abundance, Family and Order richness, Family and Order diversity, and Family and Order evenness for both benthic and drift communities. Lastly, the total number of species fo und in the community was converted into a percentage at both the Family and Order level to compare community composition. RESULTS A total of 11 sites were measured for discharge, benthic organisms , and drift organisms (Appendix, Table 1). Site eight was excluded from analysis because it was measured during a day when the rivers were flooded which is known to cause unusually high amounts of drift (Minshall and Winger 1968). Identification revealed 17 Families and 11 Orders with a total of 150 individual s in the drift community, and 26 Families and 11 Orders with a total of 368 individuals in the benthic community. Correlations Between Community and Discharge There were no significant correlations between drift community size (N), richness (S), divers ity (H'), and discharge. There was, however, a significant correlation between benthic richness and diversity, and discharge, at both the Family and Order level. Figure 3 shows the relationship of individuals and discharge in drift communities for which

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4 there was a negative trend and in benthic communities for which a positive trend was observed, neither showing a significant correlation (Spearman rank correlation, Drift: r = 0.2727, p = 0.4458, n = 10; Benthic: r = .3769, p = 0.2830, n = 10). Although a weak negative trend was observed, drift communities did not show a significant correlation between Family and Order richness and discharge (Figure 4; Spearman rank correlation, Family: r = 0.3818., p = 0.2763, n = 10; Order: r = 0.4303, p = 0.2145, n = 10). Benthic communities showed a strong, positive correlation between Family and Order richness and discharge (Figure 4; Spearman rank correlation, Family: r = 0.6525, p = 0.0409, n = 10; Order: r = 0.6420, p = 0.0454, n = 10). Drift communities did no t show a significant correlation between Family and Order diversity and discharge (Figure 5; Spearman rank correlation, Family: r = 0.0182, p = 0.9602, n = 10, Order: r = 0.0547, p = 0.8807, n = 10). Benthic communities showed a strong, positive correlat ion between Family and Order diversity and discharge (Figure 5; Spearman rank correlation, Family: r = 0.7818, p = 0.0075, n = 10; Order: r = 0.9152, p = 0.0002, n = 10). Drift and benthic communities showed no trends between Family and Order evenness and discharge, indicating that richness that controls diversity (Figure 6). Community Structure The drift community, which included 11 Orders and 17 Families, was largely dominated by the Dipteran, Trichopteran, and Ephemopteran Orders and the Simuliidae, H ydropsychidae, and Baeti dae Families (Figure 6 ). The taxa unique to drift communities were C. Arachnida, O. Megaloptera, Dipteran Families Culicidae, Nematocera, and Dolichopodidae, and Hemiptera F. Saldidae. Similar to the drift community, the benthic c ommunity was dominated by the Dipteran, Coleopteran, and Trichopteran Orders and Simuliidae, Elmidae and Hydropsychidae Families. Figure 7 shows that Diptera dominated the drifting community much more than the benthic community, Trichoptera was almost exa ctly the same in both communities, and O. Coleoptera strongly dominated in the benthic community. DISCUSSION The Effect of Discharge on Macroinvertebrate Communities The results here relating both benthic and drift communities to discharge on the Quebra da MÃ¡quina indicate that aquatic invertebrates respond to stream flow. The benthic community significantly increased in both richness and diversity as discharge increased. Because of these significant correlations, the benthic community is actively or pa ssively responding to discharge. The drift communities showed a weak negative trend to discharge. Although the correlation is not significant, it appears that the drift community is also responding to discharge. The lack of significance for the drift co mmunity could be due to a small sample size. Overall, the results prove that discharge directly or indirectly elicits a response from tropical stream macroinvertebrates.

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5 Drift and Benthic as One Community Upon assessing community structure, there is no overwhelming evidence that drift is a separate community from the benthic community. This may be due to a small sample size. A larger sample size would help distinguish what species were simply rare and which ones were unique to the drift community. O. Diptera, F. Culicidae, and O. Hymenoptera were the only taxa of the drift community that were found in large numbers and were absent from the benthic community. However, these samples were both in adult stages and had fully developed wings, which suggest s that they live just above the water and are not part of the aquatic drift community. These results suggest that the drift individuals found arise from the benthic community. Drift as an Active Behavioral Response Given that drift and benthic organisms belong to one community, their relationship with discharge can now be connected. The decreasing trend in drift individuals and the increasing correlation in benthic richness and diversity indicate that as stream flow increases, macroinvertebrates that ar e drifting become benthic. The question that remains is if this response is active or passive. If this were a passive response to discharge, one might expect the increasing velocity associated with increasing discharge to force benthic macroinvertebrates into drift. Therefore, the results from an increase in discharge would reveal a decrease in benthic numbers and an increase in drift numbers. However, this results support the opposite, which refutes the hypothesis that it is passive response. One poss ible explanation supports an active response to discharge. As mentioned before, one possible motivation in entering drift is to avoid competition (Smock 1996). An increase in stream flow may decrease drift and increase number of benthic individuals becau se as the water level becomes higher, macroinvertebrates can find more benthic surface to colonize and settle out of drift. In other words, there is not as much competition for space in areas of higher discharge. Another explanation is that, as the strea m moves faster, there is more dissolved oxygen in the water. Oxygen is a resource that macroinvertebrates seek as they drift and possibly upon finding a higher content of dissolved oxygen they would settle into the benthic community. Both of these theori es explai n why numbers of individuals drift ing decrease and individuals in the benthic community increase in areas of high discharge and explain it as an active behavioral response from macroinvertebrates. Understanding behavioral responses of tropical m acroinvertebrates is a considerable advancement in the understanding of stream ecology. Furthermore, with the comprehension that the drift and benthic communities are one community, future researchers may obtain a sample of the whole community by just sam pling drift or benthic organisms. Depending upon the results needed, the methods of sampling one habitat might be preferred over the other. Lastly, f urther research can be conducted to see if drift is similar in polluted waters as it is in pristine habit ats. This could become useful information in studies on the effects of pollution that can possibly look at drift communities as bio indicators. With knowledge of the best community to study, there is increased potential for producing significant results.

7 Figure 1. Map of location of sites on Quebrada MÃ¡quina. Samples taken f rom October 28 November 13 of 2004.

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8 Figure 2. Map of sites along Quebrada MÃ¡quina. Samples taken from October 28 November 13 of 2004. Site one was taken five meters upstream of the dam and the distance between neighboring sites is five meters. Site eight was measured but not included in analysis due to weather.

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9 Figure 3. Relationships between abundance and discharge for all s ites along the Quebrada M Ã¡ quina , Monteverde from October 28 No vember 13 of 200 4. A) Non significant negative trend between abundance of drift and discharge. B) Non significant positive trend between abundance of ben thic and discharge.

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10 Figure 4. Relationships between richness (at bot h the Family and Order level) and discharge for all sites along the Quebrada MÃ¡quina , Monteverde from October 28 November 13 of 2004. A) Non significant negative trend between drift Family richness and discharge. B) Significant positive correlation between benthic Family richness and discharge. C) Non significant negati ve trend between drift Order richness and discharge. D) Significant positive correlation between benthic Order richness and discharge.

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11 Figure 5. Relationships between diversity (at both the Family and Order level) and discharge f or all sites along the Quebrada MÃ¡quina , Monteverde from October 28 November 13 of 2004. A) Non significant between drift Family diversity and discharge. Significant positive correlation between benthic Family div ersity and discharge. B) Non significant between drift Order diversity and discharge. Significant positive correlation between benthic Order diversity and discharge. Figure 6. Relationships between evenness (at both the Family and Order level) and discharge for all sites along the Quebrada Maq uina, Monteverde from October 28 November 13 of 2004. A) Non significant relationship between both drift and benthic and discharge. B) Non significant between both drift and benthic and discharge.

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12 F igure 7. Percentages of drift versus benthic communities to Family for all sites along the Quebrada MÃ¡quina , Monteverde from October 28 November 13 of 2004.

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13 Figure 8. Perce ntages of drift versus benthic communities to Order for all sites along the Quebrada MÃ¡quina , Monteverde from Oct ober 28 November 13 of 2004.